Q&A for all webinars

Harold McNair, Virginia Tech, USA

LevelBasic
The First webinar in the year of education was held by Harold McNair: Introduction to Gas Chromatography. We decided to start with this really basic seminar because we anticipated a large demand on these basics. And we were right, over 900 attended the webinar. The live webinar has been recorded and can be viewed here.

"Introduction to GC" has also been recorded as a video presentation. Click here to go to the series of five, click here to watch a free taster.

During the webinar we received a large number of questions. Professor Harold McNair has answered them. You will find the questions and answers below:

Introduction to Gas Chromatography

     “Introduction to Gas Chromatography” Webinar
                 28 October 2009
            Question and Answers


1. For FID could I consider any area percent as concentration in wt %?	No. Area percent is not weight percent. You need to make a calibration curve with pure standards. A good example is for ECD; 1µg of benzene, toluene and xylene might generate peak areas of 1000, 3000 and 5000.You must calibrate for each compound.
2. With sample cleanup, I doubt that the quantitative aspects are guarantueed?	If you add an internal standard to the sample matrix before clean-up you will normally get better quantitative results.
3. How is Gas chromatography going to deal the eventual exhaustion of Helium? Will Hydrogen become the standard carrier gas in the future? When do we run out of Helium?	No good answer at this time. Hydrogen will be used more as carrier gas, however helium will remain the most widely used for some time.
4. My lab uses hydrogen as a carrier gas and for GC-MS and GC-FID instead of helium. Considering the flammability of hydrogen, as well as the expense, are there advantages to using hydrogen that outweigh these issues?	I see no practical advantage in using H₂ over He today.
5. What is the use /function of the wool in the injection port?	Silanized glass wool is used primarily to adsorb non-volatile components in dirty samples. It also provides a hot, large surface area to help vaporize volatiles.
6. What is the min. quantity (In Grams) for the Sample?	For capillary columns, typical sample concentrations range from nanograms up to 100 micrograms in a solvent. Typical injection volumns are 1 microliter.
7. Hallo - i want to know how can i make a quantification analysis by using SPME GC-MS injection	Too complicated to be answered here. Please check the Chromedia topics on quantitative analysis, SPME and GC-MS
8. Is there any reason why Hydrogen gas cannot be used as carrier gas when using TCD, even we are not interested in analyzing Hydrogen??	Hydrogen can be used with the TCD. It cannot be used to measure hydrogen or helium.
9. Please define column capacity.	A broader than normal peak , (I call it cigar shaped) is a good indication of an over-loaded detector or column. When retention time decreases with larger samples it usually indicates an overloaded column.
10. How can i calculate the LOD of GC-MS by using calibration series	First measure the noise level. Then make up a series of pure standards, maybe 10ppm, 1ppm, 0.1ppm etc. When the peak area is 10X noise level, that is the LOQ, limit of quantization. When the peak area is 3X noise level, that is the LOD, limit of detection. That peak is present in the sample, but you cannot report any valid concentration.
11. What can we do with compounds with high molecular weight and not volatile ? Is their is any CG coupled with protone NMR,13-C NMR and mass?	If truly non-volatile, there is nothing you can do. Molecules have to be in the vapor state to be carried through a GC column.
12. What about the column unions for capillary?	Yes, use a zero dead volume fitting, or end to end fitting of identical I.D.s.
13. The detection limit of GC can be as low as picogram. Is "picogram" meant the amount injected into GC?	No, the detection limit, LOD is the smallest amount of any component in the sample which can be detected. Please check Chromedia and textbooks on this topic.
14. What is the advantage of split injection system - Prof Deokate U.A.	As capillary columns demand a very small amount of sample injected rapidly, a split injections meets both demands. It is also very simple, open a split vent.
15. What is the cause of seeing series of siloxane peaks in my blanks example "Cyclodecasiloxane, eicosamethyl-". Is it a bleeding column or liner problem?	Siloxane peaks can come from both the septum and the column liquid phase. Use a faster septum purge (4ml/min) or a Teflon backed septum to minimize septum “bleed.” Since column bleed is proportional to the amount of liquid phase: use a shorter column, a thinner film, or a more temperature stable liquid phase. If possible use a lower column temperature.
16. Can sample be vaporized at sample point or in GC oven rather than hot injection port?	Sample can be vaporized in a heated inlet liner, or inside the column itself (on-column injector or PTV).
17. Can you explain splitless and split/splitless injection?	Too complicated for here, I refer to Chromedia, which has sections on these topics.
18. What would you recommend to replace glass wool in the SSL liner due to possible adverse activity when dealing with dirty samples?	Be sure to use “silanized” glass wool, and replace it frequently.
19. Powder or bigger chip (>1.00 mm) which one is Better?	I do not understand the question.
20. Hello, about the sampling of very volatile samples especially iso-pentane. we introduce the sample as liquid but we loose the sample at the time of injection. we got very poor peaks. thank you	Use a gas sample valve; add a restrictor to the exit end of the valve to keep the gas sample loop under high pressure, at least 2-4psi.This should keep the pentane in the liquid state.
21. What do you think is the future for GC? There are some interesting news	The future of GC is slow evolution, no more revolution( my opinion). We have worked on improving GCs for over 50 years. The columns, detectors and data systems are almost at the optima. We will see faster GC, more GC/MS and emphasis on automated sample handling.
22. Can you touch upon the difference between a PTV injection port and the S-SL injection port?	Sorry, this question is beyond my expertise. Chromedia will ask within their expert team.
23. How maintenance I mean cleaning is given to a ECD after time	Maintain the ECD at a high temperature to avoid condensation of sample or column “bleed.” Annually clean the ECD by passing H₂ as carrier gas through the hot detector overnight. Use either an empty column, or a solid phase column like silica gel which does not bleed. The hot H₂ will burn off any organic residue on the radioactive foil.
24. Which gives better resolution, thick or thin film?	Thin films give better resolution than thick films. Please refer to the Golay Equation for capillary columns.
25. For trace analysis, what are the advantages and disadvantages of using large volume injection?	Large volume injection has the advantage of better detection limits. Its disadvantage is that usually it involves a special accessory, that is extra cost and inconvenience.
26. I am carrying out a chemical reaction in solution (with dry organic solvent 20 mL) in a flask with inert atmosphere. I suspect that the reaction releases molecular hydrogen gas, however bubbles are not seen. I think that H₂ (about 0.1 mmol) is dissolved in the solvent. Could GC technique detect this H₂?	H₂ can easily be detected by GC, but not with a FID. You will need to use a TCD or MS.
27. How do you coat tubing walls with liquid phase? How do you keep the liquid phase stationary?	Making capillary columns is too complicated for the time available. Please refer to a good textbook. “Modern Practice of GC” by Grob and Berry, John Wiley 2004.
28. Do you need to split injections with a megabore column?	Some injections can be made without splitting on a thick film, megabore column, but you need to start at a low temperature and then temperature programs the column. A split injection will always give better resolutionon all capillary columns .
29. When do you using constant flow and constant pressure mode ?	Use constant pressure only with iso-thermal separations, for all other methods, including iso-thermal use constant flow.We use constant flow in all situations
30. How would you trouble shoot a signal value that increases during a temperature increase. This looks like the baseline has shot up during the final temperature stage.	Several options here. Could be column bleeding. Please check the troubleshooting section on Chromedia.